470 
ARTILLERY IN COAST DEFENCE. 
but the difference, which at 0 was zero, has become a large minus 
quantity. For this reason, in order that the tables may remain correct 
approximately for all ranges, the depression range-finder must not be 
placed more than about 200 yards to a flank of the groups. These 
differences are either calculated (which gives the best results), or 
obtained by construction (in the manner shown in regulations for 
Garrison Artillery practice) ; and tabulated ; the calculation being 
made for a medium range of, usually, 2000 yards. 
Group difference tables are only made out in multiples of 25 yards ; 
as are all other tables of range corrections ; the length of the probable 
rectangle being such that it is not worth while considering any less 
quantity. The tables show the corrections which have to be made 
between certain angles of training. There must be in each group a 
group difference table for each station of the depression range-finder. 
The method of using the table is as follows :—The Group Officer, 
who in this system chooses the actual moment to fire his guns (subject 
to the orders he may have received from the Fire Commander as to 
rate and order of firing), watches the dial by which the ranges are 
passed down to him; and, taking into account the rate at which the 
range is altering, and the time that it takes to lay the guns, makes up 
his mind to commence firing when the dial shows a certain range. He 
then looks at the training of a gun, first ascertaining that it is nearly 
on the object, notes the reading on the arc, and referring to his differ¬ 
ence table finds against that reading a certain correction; he makes 
this correction to the range he made up his mind to fire at, and gives 
the order to lay with this corrected range ; he then watches the dial 
again, and when it shows the range at which he decided to fire, he 
gives the order, “ Commence firing.” An example, perhaps, makes 
this process clearer. 
During the loading the Group Officer has ascertained that the range, 
as shown on the dial, is decreasing by 100 yards every 15 seconds; 
when the guns are run up the range dial reads 2500 yards; he knows 
that it will take 30 seconds to lay the guns, and therefore decides to 
fire when the dial reads 2300. As soon as one of the guns is nearly 
on the object (for direction) he notes that the training reads 330°, 
referring to his table he finds that between the trainings 315° and 
340°, a correction of + 75 is necessary (this means, of course, that in 
that position the object is approximately 75 yards farther from the 
group than from the depression range-finder) ; he then gives the 
order, “ 2375 yards, lay,” and as soon as the dial shows 2300 he gives 
the order, “ Commence firing.” In practice it will be found to save 
trouble if the difference corrections are chalked on the floor against 
the portion of the arc to which they refer, as the Group Officer is then 
saved the necessity of referring to the table. 
By this means the range from the depression range-finding station 
to the object is converted, at each group, into the range from that 
group by the Group Officers. 
There are, however, several other corrections which have to be 
made by the Fire Commander ; for the elevation which must be given 
to a gun in order to hit an object (which is called the relative range), 
